When is a function not an expression? It is just a special case of an expression. Anywhere where PowerShell or any other language says this syntax tag <expression> we can substitute a function. If yuo are thikning the aan expresion needs an "=" sign then you misunderstand the definition of expression.

the numerb 1 is an experssion in its simplewst form. Statign and object at a PowerShell causes evealuation and what is typed must be a legitimate expression. These are all legitimate expressions.